A suitable device is needed for the direct measurement of oxygen partial pressure (PO2) in blood and tissue for both clinical and research applications. A fiber-optic sensor is ideal for this application, with the advantage for physiological use of very small size and flexibility, safety, and low cost. An optical P02 sensor has been developed, based upon the principle of fluorescence quenching by oxygen. The feasibility of the sensor and its satisfactory performance have been demonstrated in vitro and in vivo tests in preceding years. The development of the sensor in the form of a needle probe was done, and an instrument suitable for research applications was developed. The system was used in experiments to measure P02 in the eyes of anesthetized dogs, in comparison with polarographic electrode measurements Of PO2. This work was done to evaluate the suitability of the sensor for studies of diabetic retinopathy. It resulted in the design and construction of an instrument with performance improvements suitable for further experiments in this and other projects.